1. Field of the Invention
This invention relates in general to the adaptive reduction of electromagnetic interference.
2. Description of the Prior Art
Adaptive interference cancellation techniques dynamically reduce undesirable interference signals in a wide variety of systems including communication systems, radar and sonar.
In a conventional adaptive system, the outputs from several antennas are combined through adaptive circuitry to produce a single input to a receiver. This combination is adjusted dynamically in the presence of interference signals to cancel the interference signal at the receiver input.
The operation of conventional adaptive systems may be interpreted in either the spatial domain or in the circuit domain. In the spatial domain, the adaptive system forms a null in the pattern of the receiving antenna system in the direction of the interference signal. Alternatively, in the circuit domain, the adaptive system adjusts itself to cancel the interference power at the receiver input.
Conventional adaptive interference designs have several shortcomings. One major shortcoming is that effective interference cancellation can be achieved only over a narrow bandwidth. In conventional adaptive interference designs, the output terminals of the antennas are combined with complex amplitude and phase weight settings. The interference signals received by each antenna element experience different amplitude, phase levels and time delays. In addition, these amplitude and phase levels and time delays vary with the direction of the arriving interference.
For very narrow bandwidth signals, these levels and time delays reduce to an aggregate amplitude and phase sum, and the interference can be cancelled by a simple amplitude and weight setting. However, when interference cancellation over a broader bandwidth is required by applications such as high data rate communications or high resolution radar systems, the simple amplitude and phase combination is no longer adequate to cancel interference.
A conventional technique to extend adaptive cancellation bandwidth uses fixed tapped delay line circuitry with amplitude and phase control at each tap, referred to as an "adaptive transversal equalizer," to produce a frequency dependent adaptive weight that extends the cancellation bandwidth of the adaptive cancellation system. However, fixed taps provide only marginal improvements in bandwidth since they do not precisely match the time delays inherent in the antenna system nor their variation with the direction of the arrival of the interference.
A second shortcoming of conventional adaptive interference designs is the time required to determine the adaptive weight settings used in the cancellation circuitry. The conventional adaptive designs use recursive interference correlation techniques or an iterative procedure. In practice, this process is time consuming.
One object of the present invention therefore is to provide adaptive interference cancellation across a broad bandwidth.
A second object of the present invention is to reduce the time required to adaptively cancel interference.